The general aim of this proposal is to employ gases to study different aspects of human physiology. The specific aims are as follows: 1) Measure the efficiency of luminal stirring in the small bowel of man and dogs using carbon monoxide (CO) and other compounds as probes. These studies will 1) determine if the malabsorption occurring in various forms of intestinal pathology could result, in part, from inadequate stirring of luminal contents; and 2) determine what aspect of intestinal motility induces luminal stirring. 2) Determine if the simple measurement of the concentration of CO in an expired air sample can serve as a rapid, quantitative measure of red cell life span. Such a test should have major clinical utility in the evaluation of anemia. 3) Serum (and other biological materials) were observed to spontaneously produce CO when exposed to O2. This spontaneous CO production appears to be a sensitive indicator of oxidation of some component in serum. We will investigate the nature of spontaneous CO production by human serum and determine the possible clinical implications of such CO production. 4) Fecal bacterial both produce and consume H2, and only the net of these two processes has been measured previously. In a variety of clinical situations where H2 production is thought to be important, we will independently measure total H2 production, H2 consumption, and net H2 production and determine which of these two processes has been altered. 5) The bacterial B-galactosidase in yogurt allows lactase deficient subjects to absorb lactose in yogurt more readily than lactose in milk. Using breath H2 measurements will test the ability of a modified yogurt (which contains four times the usual B-galactosidase activity) to enhance lactose absorption in lactase deficient subjects. Such a yogurt would find wide use as a natural means of enhancing lactose absorption from milk.